doc: rockchip: Update SPI flashing instruction

Update documentation on how to write a bootable u-boot-rockchip-spi.bin
image into SPI flash. This removes the reference to a hardcoded and now
obsolete 0x60000 payload offset.

Also remove an obsolete reference to pad_cat.

Signed-off-by: Jonas Karlman <jonas@kwiboo.se>
Reviewed-by: Quentin Schulz <foss+u-boot@0leil.net>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>